Apparatus and method for real time measurement of substrate temperatures for use in semiconductor growth and wafer processing

ABSTRACT

The invention is an optical method and apparatus for measuring the temperature of semiconductor substrates in real-time, during thin film growth and wafer processing. Utilizing the nearly linear dependence of the interband optical absorption edge on temperature, the present method and apparatus result in highly accurate measurement of the absorption edge in diffuse reflectance and transmission geometry, in real time, with sufficient accuracy and sensitivity to enable closed loop temperature control of wafers during film growth and processing. The apparatus operates across a wide range of temperatures covering all of the required range for common semiconductor substrates.

PRIORITY CLAIM

This application is a Continuation of U.S. patent application Ser. No.10/961,798, filed Oct. 8, 2004, hereby incorporated by reference, whichclaims the benefit of U.S. Provisional Application No. 60/509,762, filedOct. 9, 2003.

FIELD OF THE INVENTION

The invention relates to methods and devices for making precisenon-contact measurements of the temperature of substrate materialsduring the growth and processing of thin films, particularly pertainingto semiconductor growth and wafer processing.

BACKGROUND OF THE INVENTION

Precise temperature measurement during the growth of deposited layers ona semi-conductor wafer is critical to the ultimate quality of thefinished, coated wafer and in turn to the performance of theopto-electronic devices constructed on the wafer. Variations insubstrate temperature, including intra-wafer variations in temperatureultimately affect quality and composition of the layers of materialdeposited. During the deposition process, the substrate wafer isnormally heated from behind and rotated about a center axis. Typically,a resistance heater positioned in proximity to the wafer provides theheat source for elevating the temperature of the wafer to apre-determined value. Precise control of the temperature associated withthe process is most desirable, and is best achieved through precise andreal-time monitoring of the substrate temperature.

An example application illustrating the necessity of precise temperaturecontrol is the formation of semiconductor nanostructures. Semiconductornanostructures are becoming increasingly important for applications suchas “quantum dot” detectors, which require the self-assembled growth ofan array of very uniform sizes of nano-crystallites. This can only beaccomplished in a very narrow window of temperature. Temperatureuncertainties can result in spreading of the size distribution of thequantum dots, which is detrimental to the efficiency of the detector.

The growth of uniform quantum dots is an example of a thermallyactivated process in which the diffusion rates are exponential intemperature. Therefore, it is important to be able to measure, and haveprecise control over, the substrate temperature when growth orprocessing is performed.

Numerous methods have been disclosed for monitoring these temperatures.One simple, but largely ineffective approach has been the use ofconventional thermocouples placed in proximity to, or in direct contactwith the substrate during the thin film growth operation. Thismethodology is deficient in many respects, most notably, the slowresponse of typical thermocouples, the tendency of thermocouples (aswell as other objects within the deposition chamber) to become coatedwith the same material being deposited on the semi-conductor wafer,thereby effecting the accuracy of the thermocouple, as well as the spotthermal distortion of the surface of the semiconductor wafer resultingfrom physical contact between the thermocouple and the substrate. In anyevent, the use of thermocouples near or in contact with the substrate islargely unacceptable during most processes because of the poor accuracyachieved.

Optical pyrometry methods have been developed to overcome theseshortcomings. Optical pyrometry uses the emitted thermal radiation,often referred to as “black body radiation,” to measure the sampletemperature. The principal difficulties with this method are thatsamples typically do not emit sufficient amounts of thermal radiationuntil they are above approximately 450° C., and semiconductor wafers arenot true black body radiators. Furthermore, during deposition asemiconductor wafer has an emissivity that varies significantly both intime and with wavelength. Hence the use of pyrometric instruments islimited to high temperatures and the technique is known to be prone tomeasurement error.

In “A New Optical Temperature Measurement Technique for SemiconductorSubstrates in Molecular Beam Epitaxy,” Weilmeier et al. describe atechnique for measuring the diffuse reflectivity of a substrate having atextured back surface, and inferring the temperature of thesemiconductor from the band gap characteristics of the reflected light.The technique is based on a simple principle of solid state physics,namely the practically linear dependence of the interband opticalabsorption (Urbach) edge on temperature.

Briefly, a sudden onset of strong absorption occurs when the photonenergy, hv, exceeds the bandgap energy E_(g). This is described by anabsorption coefficient,

α(hv)=α_(g)exp[(hv−E _(g))/E ₀],

where α_(g) is the optical absorption coefficient at the band gapenergy. The absorption edge is characterized by E_(g) and anotherparameter, E₀, which is the broadening of the edge resulting from theFermi-Dirac statistical distribution (broadening ˜k_(B)T at the moderatetemperatures of interest here). The key quantity of interest, E_(g), isgiven by the Einstein model in which the phonons are approximated tohave a single characteristic energy, k_(B). The effect of phononexcitations (thermal vibrations) is to reduce the band gap according to:

E _(g)(T)=E _(g)(0)−S _(g) k _(B)θ_(E)[exp(θ_(E) /T)−1]

where S_(g) is a temperature independent coupling constant and θ_(E) isthe Einstein temperature. In the case where θ_(E)>>T, which iswell-obeyed for high modulus materials like Si and GaAs, one canapproximate the temperature dependence of the band gap by the equation:

E _(g)(T)=E _(g)(0)−S _(g) k _(B) T,

showing that E_(g) is expected to decrease linearly with temperature Twith a slope determined by S_(g) k_(B). This is well obeyed in practiceand is the basis for the band edge thermometry.

Variations on this methodology are taught by Johnson et al., in U.S.Pat. No. 5,388,909, and U.S. Pat. No. 5,568,978. These references teachthe utilization of the filtered output of a wide spectrum halogen lampwhich is passed through a mechanical chopper, then passed through alens, then through the window of high vacuum chamber in which thesubstrate is located, and in which the thin film deposition process isongoing. Located within the chamber is a first mirror which directs theoutput of the source to the surface of the substrate. The substrate isbeing heated by a filament or a similar heater which raises thetemperature of the substrate to the optimum level required for effectiveoperation of the deposition process. A second mirror located within thechamber is positioned to reflect the non-specular (i.e., diffuse) lightreflected from the back surface of the substrate, said reflection beingdirected to another window in the chamber and thence through a lens to adetection system comprising a spectrometer. The wavelengths of theelements of the non-specular reflection are utilized to determine theband gap corresponding to a particular temperature. Johnson et al. teachthat the temperature is determined from the “knee” in the graph of thediffuse reflectance spectrum near the band gap.

While the prior art is in some ways effective, use of optical fiberbundles, intra chamber optics, mechanical light choppers andmechanically scanned spectrometers renders the methodology deficient inmany respects. The detected signal suffers from temporal degradation ofthe optics within the deposition chamber. The mechanical components areoverly susceptible to failure and the overall methodology of collectingthe signal is simply too slow for real-time measurement and controlapplications in the industrial production environment. In addition, thedescribed means of the prior art is subject to variations in accuracydependent upon the fluctuation, over time, of the output of the halogenlight source.

Specifically, the prior art relies on one or more optical elementswithin the deposition chamber to direct the incident light to the waferand to collect the diffusely reflected light. The presence of opticswithin the deposition chamber is problematic, since the material beingdeposited during the coating process tends to coat all of the contentsof the chamber, including the mirrors, lenses, etc. Over time thecoatings build up and significantly reduce the collection efficiency ofthe optics and can lead to erroneous temperature measurement.

More importantly, the prior art relies on a mechanical light chopper anda mechanical scanning spectrometer for measurement of the light signal.Not only do the mechanical components fail frequently with extended use,but it is well known that gears in scanning spectrometers wear,resulting in continual shifts in the wavelength calibration. This leadsto perpetually increasing errors in temperature measurement unless theinstrument is recalibrated frequently, which is a very time consumingprocess. In addition, it is well known that scanning spectrometers arequite slow, requiring anywhere from 1-5 seconds to complete a singlescan. In most deposition systems the semiconductor wafers are rotating,typically at 10-30 RPM. In this case, a temperature measurement thattakes 1-5 seconds to complete is by default an average temperature andit is impossible to make any type of spatially resolved measurement. Ifthe process chamber has many wafers rotating on a platter about a commonaxis, as is typical in a production deposition system, the slow responsetime of the prior art makes it impossible to monitor multiple wafers.

Furthermore, the prior art utilizes a quartz halogen light source withno consideration of any type of output stabilization or intensitycontrol. Quartz halogen lamps are known to degrade rapidly over timeleading to fluctuations in the lamp output that result in measurementvariations and further system downtime for lamp replacement.

Basically, the many limitations of the prior art have limited theapplications of diffuse reflectance or “band edge” thermometry in thecommercial setting.

BRIEF SUMMARY OF THE INVENTION

The invention is an optical method and apparatus for measuring thetemperature of semiconductor substrates, in real-time during thin filmgrowth and wafer processing, utilizing the nearly linear dependence ofthe interband optical absorption edge on temperature.

The present invention utilizes simple, efficient collection optics,external to the deposition system, connected via a single small coreoptical fiber to a solid state array spectrometer. The system requiresno mechanical light chopper or other means to modulate the light signal.The invention can operate in one of three modes: 1.) the above describeddiffuse scattering reflectance mode, by utilizing a unique feedbackcontrolled, stabilized light source that has all optics completelyexternal to the deposition system. 2.) transmission mode with externallight source or 3.) transmission mode utilizing the substrate heater asa light source (requiring no external light source).

The invention utilizes sophisticated software algorithms to analyzediffusely scattered light from the semiconductor substrate to accuratelyand precisely determine the wavelength position of the opticalabsorption edge. The measured position of the absorption edge iscompared to calibration data using a multi-order polynomial equationthat is specific to each semiconductor wafer material. The dataacquisition speed and software algorithms are fast enough to providetypical temperature sampling rates of 20 Hz or better. The inventionoperates across a wide range of temperatures covering all of therequired range for growth on common substrates, including GaAs, Si, InP,ZnSe, and other semiconductor wafers. In particular, the system designis optimized for the temperature regime between ambient and ˜700° C.that is not currently served by existing non-contact sensors (e.g.,pyrometer-type sensors).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of one embodiment of the invention depicting thelight source and detector, and the other major components of the system.

FIG. 2 is a top perspective view of the variable focus quartz halogenlight source with the respective placement of the optics, components,and light intensity feedback sensor.

FIG. 3 is a top perspective view of one embodiment of the detectorassembly, that which utilizes lenses for collection and imaging thediffusely scattered light, showing the respective placement of theoptics and collection fiber.

FIG. 4 is a top perspective view of a second embodiment of the detectorassembly, which utilizes a single focusing mirror for collection andimaging of the diffusely scattered light, showing the respectiveplacement of the mirror and collection fiber.

FIG. 5A is a graph showing the raw, unprocessed, diffusely scatteredlight spectra from a typical semiconductor wafer at severalpredetermined wafer temperatures demonstrating the wavelength dependenceof the absorption edge on temperature.

FIG. 5B is a graph showing the spectra of FIG. 5A after the spectra havebeen preprocessed to remove background light below the absorption edgeand normalize the maximum intensity.

FIG. 6 is a graph showing the diffusely scattered light spectrum afterit has been fully processed and a linear fit has been performed in theregion of the absorption edge to determine the exact absorption edgewavelength.

FIG. 7 shows the spectra processing configuration dialog from thesoftware user interface.

FIG. 8 is graph showing the typical measured relationship for theabsorption edge wavelength position versus wafer temperature measured bya thermocouple in direct contact with the surface of the wafer.

FIG. 9 is a graph showing a multi-order polynomial fit to the absorptionedge wavelength position versus sample temperature data.

FIG. 10 is a graph showing the long term stability of the temperaturemeasurement apparatus at a single predetermined wafer temperature.

FIGS. 11A and 11B are simplified schematic drawings of the apparatus ina multi-wafer deposition system demonstrating the geometry formeasurement of multiple wafers on a rotating wafer platen.

FIG. 12 is a graph showing typical temperature data obtained from arotating platen of multiple semiconductor wafers in a multi-waferdeposition system.

FIG. 13 is a detailed graph showing the variation in temperature acrossthe surface of multiple wafers in a multi-wafer deposition system aftera single rotation.

FIG. 14 is a graph showing the measured band-edge wafer temperature as afunction of time as the wafer set point temperature is first set to 300degrees Celsius, then 450 degrees Celsius.

FIG. 15 is a schematic of a second embodiment of the invention depictingthe substrate heater as the light source and the detector, intransmission geometry, in relation to a deposition chamber.

FIG. 16 is a schematic of a third embodiment of the invention depictingthe detector, in transmission geometry, in relation to an external lightsource.

DESCRIPTION OF THE EMBODIMENT

A schematic of one embodiment of the measurement apparatus 10, depictingthe light source 12 and detector assembly 26 in diffuse scatteringreflectance geometry, in relation to a deposition chamber 16, is shownin FIG. 1. The system comprises a broad band light source 12 mounted inproximity to a transparent view port 18 on the chamber. The light source12 is typically a quartz halogen lamp, mounted outside the depositionchamber 16 which illuminates a semiconductor wafer 20 (ghost lines inthis view) from its front (polished) surface 22. The apparatus alsocomprises a detector assembly 26, also mounted outside the depositionchamber 16 proximate to a transparent view port 18 at an angle that isnon-specular to the light source 12; an optical fiber assembly 27,including a first optical fiber 28 coupled to an array spectrometer 32,and a second optical fiber 30 running collinear to first optical fiber28 and coupled to a visible alignment laser 34 for aid in alignment ofthe detector assembly 26. The optical components are optimized, usingappropriate optical coatings, for either infrared or visible operationdepending on the characteristics of the wafer 20 being measured. Thelight source 12 is connected to control assembly 35, containing lightsource power and control unit 36 via light source power/data cable 38.Computer control of the light source 12, alignment laser 34 andspectrometer 32 is maintained by computer 40 which is connected to lightsource 12, alignment laser 34 and spectrometer 32 by USB cable 42.

Typically the back surface 24 of a semiconductor wafer 20 is opticallyrough and can act as a diffuse scattering surface for the light source12. If both sides of the wafer 20 are polished, which is sometimes thecase, a diffuser (e.g., pyrolytic boron nitride) can be inserted betweenthe back surface 24 of the wafer 20 and the substrate heater 46 toenhance diffuse scattering, but this is not a requirement. Light isdiffusely scattered from the surfaces 22, 24 and from within the bulk ofthe wafer 20, a portion of which light is scattered in the direction ofthe detector assembly 26, and is imaged onto the entrance face of theoptical fiber 28. The light is analyzed by the solid state arrayspectrometer 32.

The first step in operation of the invention is to optimize the opticsconfiguration (light source, collection optics and spectrometer) for thewavelength range required for the wafer substrate material. FIG. 2 is atop perspective view of the variable focus, 150 W quartz halogen lightsource 12 with the respective placement of the optics, components, andlight intensity feedback sensor 54. The light source 12 components aremounted to enclosure 48. The light source 12 is optimized for eithervisible or infrared output depending on the substrate material to bemeasured. This involves selecting an appropriate bulb 50 for the sourcewith either an enhanced visible or infrared coating on the lampreflector 52. The bulbs 50 are readily available from several vendors,as are suitable infrared collimating reflectors. In the preferredembodiment, additional coatings, such as gold coatings for infraredoptimization, are added to the lamp reflector 52. The light source 12 isdriven by a computer-controlled 200 W power supply 36 with an integratedfeedback control circuit that is connected to a light feedback sensor 54mounted in the vicinity of the bulb 50. The purpose of the sensor 54 andfeedback control circuit is to maintain the bulb output at a constantvalue, variable by computer 40 control, for the duration of the bulblifetime. Without the feedback control circuit the output of the bulb 50exhibits oscillations and the overall output slowly decreases over thelifetime of the bulb 50. A variable aperture 56 within the light source12 controls the size of the spot of light illuminating the semiconductorwafer 20. The light is focused onto the wafer 20 using a pair of lenses,one fixed lens 58, the other variable focusing lens 60 variable inposition to obtain the best focus at the wafer surface. The depth offield is sufficient for use on most deposition systems. The lenses 58,60 are coated with a broadband antireflection coating to minimize backreflections, hence maximizing the output of the light source. Because ofthe high heat output of acceptable bulb/reflector combinations, a fan 62is providing for cooling the light source 12.

Shown in FIG. 3 is a top perspective view of one embodiment of thedetector assembly 26. The assembly 26 mounts outside of the depositionsystem to a transparent view port 18 on the chamber 16, allowing theoptics to remain clean and uncoated. The components of the detectorassembly 26 are mounted to frame 63. A first detector lens 64 collectsthe diffusely scattered light and collimates it into the second detectorlens 66. The second lens 66 images the light onto the optical fiberassembly 27 containing single-core optical collection fiber 28. Thelenses 64, 66 are coated with a broadband anti-reflection coating tominimize signal loss at the lens surfaces. The position of the secondlens 66 can be adjusted to obtain the best focus at the fiber face 68.The optical fiber 28 can also be positioned, utilizing an adjustor 72,in x, y and z directions to assist in maximizing the amount of lightcollected into the fiber. This particular embodiment of the detectorassembly 26 also comprises a micrometer-actuated, single-axis tiltmechanism 70 built into the front of the assembly 26 to assist inpointing the detector at the wafer 20 within the chamber 16.

A second embodiment of the detector assembly 26 a, shown in FIG. 4, usesa short focal length focusing mirror 74 mounted to support 75 to collectand focus the diffusely scattered light onto the first optical fiberassembly 27. This detector assembly 26 a design also mounts outside thedeposition chamber 16 and the coatings on the mirror 74 are optimizedfor the wavelength range required for the particular substrate material.The advantage of using a mirror 74 is that reflection losses from thesurfaces of lenses are eliminated completely and all wavelengths oflight are focused to same point, thus maximizing the collectionefficiency. The disadvantage is that the overall size of the detectorassembly is larger.

The single small core optical fiber 28 component within fiber opticassembly 27 used to connect the detector assembly 26 or 26 a to thespectrometer 32 eliminates many of the shortcomings of the present fiberbundle methods and apparatus in use. It is well known that fiber bundleshave significant optical losses which are associated with the emptyspaces which exist between adjoining fibers within the same bundle.Further, the existence of multiple fibers increases the susceptibilityof the bundle to interference from stray light. It is equally well knownthat optical fibers have a predetermined “acceptance angle” and thateconomically practical optical fibers generally have a predeterminedacceptance angle with a tolerance of + or −2 degrees. While thesetolerances are satisfactory in the case of single fiber optics, opticalfiber bundles containing dozens of individual optical fibers and aremuch more susceptible to stray light, with the susceptibility increasingas the number in the bundle increases. The most important advantage of asingle fiber is the spatial selectivity afforded by their small aperture(˜400 μm). This is important for stray light rejection. Additionally,optical fiber bundles are relatively expensive, typically in a range of$300 to $400 per foot. Single optical fiber of approximately 400 microncross-section, on the other hand, costs less than $10 per foot.

With reference to FIG. 1, the fiber optic assembly 27 used in theinvention is a dual, bifurcated silica/silica fiber selected for maximumtransmission in the wavelength range required by the particularsemiconductor material. One optical fiber core of the bifurcated fiberis used for collecting light from the lenses within the detector. Theother optical fiber core is used to transmit laser light from a redvisible semiconductor diode alignment laser 34 to the semiconductorwafer 20, for use in alignment of the detector assembly 26. When thedetector assembly 26 is first attached to the deposition system, thealignment laser 34 can be activated to produce a visible red laser spotilluminating the region where the detector assembly 26 is aimed. The usea small single core fiber for light detection allows for very preciseselectivity of the region or spot on the wafer 20 for the temperaturemeasurement. The detector optics image an area of the wafer surface. Themagnification of the system is defined by the focal length of the lensesand the position of the second (variable position) lens. The image ofthe wafer 20 at the face of the optical fiber is much larger than thediameter of the core. This allows the system to spatially resolvetemperature across the wafer surface by either rotating the wafer or bymoving the position of the fiber using the x,y adjustment within thedetector assembly 26. Although it is not incorporated into the detectorassembly 26 shown, it is possible to use automated actuators to scan thex,y-position of the fiber to create a 2-dimensional map of the wafer 20surface temperature.

A principal component to realizing this invention is the very sensitive,fiber-coupled solid state array spectrometer 32. Solid state arrayspectrometers (having no moving parts) are becoming common inapplications where speed and sensitivity are essential. Their drawbackis modest resolution (˜few nm in wavelength). This is not a limitationhere, because the band-edge features are relatively broad and can bedetermined by fitting procedures to much greater precision than thespectrometer resolution. The use of a fiber-coupled array spectrometer32 for this application has the following advantages:

-   -   a. Speed: array spectrometers measure typically 128-2048        wavelength channels simultaneously. Millisecond measurement        times are possible.    -   b. Sensitivity: array spectrometers are very compact, promoting        high light throughput (low numerical aperture: F1.8-F3.0 is        typical). For InGaAs arrays, 1000 ADU/sec/picowatt at 1200 nm is        a typical sensitivity.    -   c. Wide spectral range: with careful selection of the        spectrometer grating one can cover the entire spectral range        required for this application (typically a wavelength range of        ˜300 nm would cover a temperature range from ambient to ˜700°        C.).    -   d. Infrared sensitivity: the most challenging aspect of        band-edge thermometry concerns those semiconductors with small        band gaps, in the infrared region. Commercial array        spectrometers with InGaAs photo diode arrays have recently        become available at reasonable cost. Conventional InGaAs arrays        extend the spectral range beyond that offered by conventional Si        CCD arrays (˜250-1100 nm) up to 1700 nm. This opens up a wider        range of semiconductors to band-edge thermometry.    -   e. Spatial selectivity: when used with fiber-optic coupling,        array spectrometers have excellent rejection of stray light        signals. This is because the fiber core can range from 50 um to        800 um (matched to the spectrometer numerical aperture).        Therefore, by imaging the light scattered from the illuminated        portion of the wafer 20 onto the fiber entrance core, it is        possible to eliminate stray light that originates elsewhere in        the vacuum chamber (hot evaporation sources, gauge filaments,        etc.).        The array spectrometer 32 used in this invention has sufficient        speed and sensitivity and to allow the collection of complete        spectra from the semiconductor wafer 20 at typical data rates of        20 Hz and can exceed 50 Hz if required.

Shown in FIG. 5A is an example of diffuse reflectance spectra collectedfrom a semiconductor wafer 20 at four different predeterminedtemperatures. The spectra as shown are unprocessed, “raw” spectra. Theband edge absorption is clearly visible at each temperature. Shown inFIG. 5B are the same spectra after they have been pre-processed bysoftware routines to remove unwanted background light below theabsorption edge and normalize the maximum intensity. An example of afully processed spectrum showing a linear fit to the absorption edge isshown in FIG. 6. The fit to the linear portion of the absorption edge inthe spectra is extrapolated back to the x-axis to provide a highlyaccurate and reproducible wavelength value for the band-edge. Thiswavelength value is then correlated to the sample temperature.

The software algorithms used to process the spectra and correlate theband-edge wavelength to a temperature can be dependent on the type ofsemiconductor wafer material as well as the specific geometry of thedeposition chamber. Every deposition chamber is slightly different andcan produce different artifacts into the raw spectra signal. Thesoftware processing algorithms must be flexible to handle manyapplications. Shown in FIG. 7 is the Spectra Processing Software Dialogfrom the system software. The specific steps in the spectrapreprocessing and final absorption band-edge computation processing aredescribed below.

Preprocessing:

-   -   Noise Floor: allows the system to be configured to ignore a        specific level of light deemed noise based on experimental        conditions. If no portion of the current spectrum is above the        noise floor, the system ignores the spectrum and collects        another spectrum.    -   Clip spectra: removes expected anomalies in data beyond the        absorption band-edge and provides a consistent wavelength        position for normalizing the spectra.    -   Divide data point by reference: divides a reference lamp        spectrum from the collected spectrum to remove any unwanted        features introduced by the lamp.    -   Remove Background: using derivative calculations, the parameters        under this heading configure how the system will remove black        body radiation or other unwanted light from each collected        spectrum. The derivative of a spectrum is first smoothed to        enhance broad features and remove narrow features. The point of        interest within the derivative is then determined by one of two        methods,    -   1) a linear fit to the peak of the 1^(st) derivative that        satisfies a specified height; or 2) an offset from the peak of        the 2^(nd) derivative. The wavelength of this POI is used to        find the background level of light. This background level is        then subtracted from the spectrum.    -   Clip data point to min.: all wavelength data below the        wavelength with the minimum intensity is set to the minimum        intensity value. This creates a flat line up to the wavelength        with the minimum value.    -   Subtract data point offset: subtracts the minimum intensity        value determined in the previous step from the entire spectrum.    -   Compute Bandedge Preprocessed spectra are smoothed further to        enhance broad features and remove narrow features. The        absorption edge is then computed in one of two ways; 1) the        x-intercept using a linear fit at the wavelength position of the        peak of the 1^(st) derivative, or 2) wavelength position of the        peak of the 1^(st) derivative.        The preprocessing steps outlined above allow the system to        accurately and reproducibly determine an absorption band edge        wavelength from a given spectrum. This wavelength is then        correlated to a wafer temperature through the use of calibration        files. Calibration data is obtained by collecting spectra from        semiconductor wafers at well known temperatures. The temperature        of the wafer 20 is measured by a thermocouple in direct contact        with the wafer surface. A typical calibration data file, shown        in FIG. 8, depicts the absorption band-edge wavelength versus        thermocouple (TC) temperature. The wavelength versus TC        temperature plot is slightly non-linear at low temperature but        becomes very linear, as predicted, at high temperature. Shown in        FIG. 9 is the third order polynomial fit to the data with the        polynomial coefficients computed and displayed at the top of the        graph. The second and third order polynomial coefficients are        quite small. The software uses the computed polynomial to relate        the computed absorption band-edge wavelength to a wafer 20        temperature.

The absorption wavelength versus temperature calibration depends notonly on the semiconductor material, e.g. Si, GaAs, InP, but also verystrongly on the wafer 20 thickness, dopant type, and dopant density.This requires that calibration files must be acquired for wafers 20 ofdifferent thickness, dopant type, and dopant density. Once calibrationfiles have been acquired for several variations that establish a trend,for example the shift in absorption edge due to wafer 20 thickness, thesoftware can compute calibration curves for modifications. When theproper calibration file is selected, corresponding to the correct wafermaterial, wafer thickness and dopant density, the system can preciselyand reproducibly measure the wafer temperature with high accuracy. Shownin FIG. 10 is a long term stability plot for repeated measurement of asemiconductor wafer 20 over a four hour period. The wafer 20 was held at200.0+/−0.1 degrees Celsius using a PID temperature controller with acalibrated thermocouple mounted directly to the wafer 20 surface. Theplot shows that the absorption band-edge measurement was repeatable witha maximum error of 0.1 degrees Celsius and a standard deviation over afour hour period of 0.04 degrees Celsius.

FIGS. 11A and 11B show a typical application of the invention to amulti-wafer production deposition system. Multiple wafers 20 are mountedon a platen 82 that rotates about a central axis 80. The light source 12is positioned on the outside of chamber 16 so that as the platen 82rotates, each wafer 20 individually passes beneath the light source 12.The diffusely scattered light 100 is detected from a port of chamber 16.Platen 82 rotation speeds can be as high as 60 RPM resulting in eachwafer 20 being illuminated by the light source 12 for as little as 50ms. The measurement speed of the invention is thus essential if everywafer 20 is to be measured with each rotation. An example of actualtemperature data from a commercial production deposition system is shownin FIG. 12. As shown in FIG. 11B, the wafer platen 82 holds 4, 6-inchdiameter wafers 20 and the invention is measuring each wafer 20 on theplaten 82 repeatedly as the platen 82 rotates. Each wafer temperature isshown to be highly repeatable and if the data is analyzed in detail, asshown in FIG. 13, it can be seen that the invention can spatiallyresolve the temperature across each wafer 20. The measurement shows thatsome wafers have a much larger temperature gradient than others. Onewafer 20 is much hotter at the center while another is much hotter atthe edges. This type of temperature non-uniformity can cause significantdifferences in device performance depending on where the deviceoriginates from the wafer 20.

The described invention has sufficient speed and accuracy that theband-edge wafer 20 temperature signal can be used as an input to aproportional-integral-differential (PID) control loop for the purpose ofcontrolling the output power of the substrate heater. Shown in FIG. 14is a graph of the wafer 20 temperature as a function of time, measuredusing the band-edge absorption signal in a direct feedback loop to a PIDcontroller. The temperature ramps and stabilizes very quickly to the setpoint values of 300 degrees Celsius and 450 degrees Celsiusrespectively.

In further embodiments of the invention, the system utilization can beextended by operating the system in transmittance rather thanreflectance geometry. In a third embodiment of the invention, shown inFIG. 16, an external light source 12 can be mounted to illuminate eitherthe front or back side of the wafer 20 and the detector assembly 26 canbe mounted on the opposite side of the wafer 20 in a transmissiongeometry. In some applications where there is limited space behind thesubstrate heater 46, a quartz rod can be placed behind the wafer 20 tocollect and redirect the transmitted light 98 to a suitable port wherethe detector can be mounted. Provided the quartz rod is located behindthe wafer 20, it will not be coated by the deposition process. FIG. 16is a schematic of a third embodiment of the invention depicting thedetector assembly 26, in transmission geometry, utilizing the substrateheater 46, within the deposition chamber 16, as the source of light. Inthis geometry no external light source is required.

In conclusion, a new real-time, non-contact temperature measurementsystem has been described—for use in semiconductor growth and waferprocessing applications. The invention is designed to overcome thelimitations of existing technology to provide a versatile non-invasivetemperature sensor for a much wider set of applications in the thin filmsemiconductor arena. Taking advantage of recent developments infiber-coupled array spectrometers, the new invention provides a powerfultool to characterize multi-wafer temperature uniformity in productionreactors, a measurement that cannot be performed currently with othertemperature measurement techniques. Numerous obvious modifications maybe made to the invention without departing from the scope thereof.

1. A method for determining the temperature of a semiconductor materialby spectral analysis in an environment where there are light signalsfrom wanted and unwanted sources, said method comprising the steps of:a) providing a semiconductor material; b) interacting light signals withthe semiconductor material to produce diffusely scattered light; c)collecting light in a spectrometer to produce spectra data by resolvinglight signals into discrete wavelength components of particular lightintensity, the light containing both diffusely scattered light from thesemiconductor material along with a component of unwanted light signals;d) identifying an absorption edge region in the spectra data; e)deriving a band edge wavelength value as a function of the identifiedabsorption edge region; f) inferring a temperature of the semiconductormaterial based on the derived band edge wavelength value; g) andsubtracting the unwanted light component from the spectra data beforesaid step of deriving a band edge wavelength to create preprocessedspectra, whereby the temperature of a semiconductor material can bedetermined without modulating the wanted light signal prior to saidcollecting step.
 2. The method of claim 1 wherein said step ofsubtracting the unwanted light component includes determining a point ofinterest wavelength within the spectra data using a derivativecalculation.
 3. The method of claim 2 wherein said step of determiningthe point of interest wavelength includes the steps of: a) calculating a1^(st) derivative of the spectra data; b) identifying a peak feature ofthe 1^(st) derivative within the absorption edge region; c) making alinear fit to the absorption edge region at the wavelength position ofthe 1^(st) derivative peak; d) extrapolating the linear fit to determinea wavelength position for subtraction of the unwanted light component.4. The method of claim 2 wherein said step of determining a point ofinterest wavelength includes the steps of: a) calculating a 2^(nd)derivative of the spectra data; b) identifying a peak feature of the2^(nd) derivative within the absorption edge region; c) using thespectra intensity value at the 2^(nd) derivative peak, or the intensityvalue just prior to the 2^(nd) derivative peak, as the unwanted lightcomponent to subtract.
 5. The method of claim 1 wherein said step ofderiving the band edge wavelength value includes the steps of: a)calculating a 1^(st) derivative of the preprocessed spectra; b)identifying a peak feature of the 1^(st) derivative within theabsorption edge region; c) making a linear fit to the absorption edgeregion at the wavelength position of the 1^(st) derivative peak; d)extrapolating the linear fit to determine the bandage wavelength.
 6. Themethod of claim 1 wherein said step of deriving a band edge wavelengthvalue includes: a) calculating a 2^(nd) derivative of the preprocessedspectra; b) identifying a peak feature of the 2^(nd) derivative withinthe absorption edge region; c) using the 2^(nd) derivative peakwavelength position as the band edge wavelength.
 7. The method of claim1 further including the step of constructing a calibration function ofband edge wavelength versus inferred semiconductor material temperature,and wherein said step of inferring a temperature of the semiconductormaterial includes correlating the band edge wavelength value to atemperature using a calibration function.
 8. The method of claim 7wherein said step of constructing a calibration function includescollecting spectra data from semiconductor materials at predeterminedtemperatures.
 9. A method for determining the temperature of asemiconductor material via spectral analysis in an environment wherethere are light signals from wanted and unwanted sources, said methodcomprising the steps of: h) providing a semiconductor material; i)interacting light signals with the semiconductor material to producediffusely scattered light; j) collecting light in a spectrometer toproduce spectra data by resolving light signals into discrete wavelengthcomponents of particular light intensity, the light containing bothdiffusely scattered light from the semiconductor material along with acomponent of unwanted light signals; k) identifying an absorption edgefeature in the spectra data; l) ignoring a predetermined level of lightdeemed noise based on experimental conditions; m) clipping the spectradata; n) dividing a reference lamp spectrum from the collected light toremove any unwanted features introduced by a lamp; and o) removingunwanted light from the spectra data to create preprocessed spectra. 10.The method of claim 9 further including the steps of: p) creating a flatline up to the wavelength with the minimum value by setting a minimumintensity value; and q) subtracting the minimum intensity value from thespectra data to establish a data point offset.
 11. The method of claim 9wherein said step of clipping the spectra data includes removinganomalies in the spectra data beyond the absorption edge and providing aconsistent wavelength position for normalizing the spectra data.
 12. Themethod of claim 9 wherein said step of removing unwanted light includesusing a derivative calculation.
 13. The method of claim 12 wherein saidstep of using a derivative calculation includes smoothing the spectradata to enhance broad features and remove narrow features.
 14. Themethod of claim 13 wherein said step of using a derivative calculationincludes determining a point of interest within the derivative.
 15. Themethod of claim 14 wherein said step of determining the point ofinterest wavelength includes the steps of: a) calculating a 1^(st)derivative of the spectra data; b) identifying a peak feature of the1^(st) derivative within the absorption edge region; c) making a linearfit to the absorption edge region at the wavelength position of the1^(st) derivative peak; d) extrapolating the linear fit to determine awavelength position for subtraction of the unwanted light component. 16.The method of claim 14 wherein said step of determining a point ofinterest wavelength includes the steps of: a) calculating a 2^(nd)derivative of the spectra data; b) identifying a peak feature of the2^(nd) derivative within the absorption edge region; c) using thespectra intensity value at the 2^(nd) derivative peak, or the intensityvalue just prior to the 2^(nd) derivative peak, as the unwanted lightcomponent to subtract.
 17. The method of claim 9 further including thesteps of: r) further smoothing the spectra data to enhance broadfeatures and remove narrow features; s) computing a band edge wavelengthvalue; and t) inferring a temperature of the semiconductor materialbased on the computed band edge wavelength value.